With the rapid advancement of computer technology, there has been a concomitant development of improved printing technologies. In addition to printing methods such as letterpress, offset, lithography, flexography and rotogravure, advanced electronic printing systems have emerged. In electronic printing systems, information is transmitted from a computer to a printing instrument in digital form. This format allows for the use of more sophisticated printing devices than the traditional formed character impact system such as the "daisy wheel" printer. Examples of such non-impact printing technologies currently in use or development are: laser xerography, thermal, thermal transfer, electrostatic and ink jet. These methods form printed images by "bit-mapping" wherein the image is formed by discrete dots separately addressable so that text and graphical information can be intermixed in a single printed document. Since these methods do not rely on the impact between a character forming element and an inking ribbon, they can operate more quietly and at a higher speed than former printing methods.
Printing systems, in particular non-impact electronic technologies, necessitate appropriate materials such as papers and ink compositions to produce a high quality product. One type of modern electronic printing instrument is the ink jet printer. Ink jet printers include continuous and so-called "drop-on-demand" ink-jet systems, both of which emit droplets of ink under pressure from a nozzle. In the continuous jet systems, ink is ejected in a continuous stream, and the ink that is not used for printing is recirculated. In this system, an electro-mechanical transducer vibrates to break up the steady flow of ink into droplets which are electrically charged and correspond in amount to the signal strengths for the shape of the characters. Drop-on-demand ink jet systems do not recycle ink and rely on piezo-crystal formation to generate a spurt of high pressure to force out drops of ink onto the paper. The bubble-jet system, typified by the Hewlett Packard "Thinkjet.RTM." printer, is a variation of the drop-on-demand system, where heat is used to generate bubbles that create a brief pulse of high pressure that propels ink out of a chamber onto the paper. For printing purposes, it is necessary that drops be uniform in size, equally spaced from each other, and be formed at a high rate. Z. Kovac and C. Sambucetti, Magnetic Ink for Magnetic Ink Jet Printing, Colloids and Surfaces in Reprographic Technology, 1982, the disclosure of which is incorporated herein by reference.
In the past, the driving method and structure of the printing head in ink jet printers have caused difficulties in achieving acceptable results (U.S. Pat. No. 4,314,259) and have limited the type of ink formulations which will work properly. Optimal print quality is a function of the physical properties of the ink composition such as surface tension and viscosity. Ink jet printer inks are typically formed by dissolving or dispersing dyes or pigments in solvent with additives such as antiseptics and stabilizers. Technical problems encountered with these solvent-based inks include those due to interactions between the ink and the paper surface: after striking the paper, the solvent used in the ink drop spreads before drying. Dye dissolved in the solvent spreads along with the solvent, resulting in a "feathering" effect which degrades resolution since adjacent dots of ink may overlap. Moreover, the dyes in the ink may separate chromatographically in this process, causing an undesirable effect. The solvent and dye also tend to penetrate into the paper. This can result in "print through", where a shadow of the image is observed on the reverse side of the paper sheet. Two-sided printing is not possible under these circumstances.
Previously, the above problems have been treated by modifying the surface of the paper. Typically, the paper used in ink jet printing is heavily coated with a variety of materials such as clays designed to reduce or control the spreading of the ink. These coatings, however, increase the cost of the paper and often reduce its aesthetic appeal. In addition, coatings which work well with a particular ink jet may not perform well with other marking instruments used in printing apparatus, such as pens or pencils. Alternatively, particulate inks have been used to eliminate feathering, but have been found to settle out of the ink solution over a long period of time and tend to clog the printer nozzle.
In general therefore, there is a need for new and improved ink compositions. Specifically, there is a need for improved ink compositions enabling images to be found that have excellent resolution and improved waterfastness and lightfastness. Water-fastness can be defined as that property of the ink composition which renders it resistant to removal or spreading on the paper when exposed to water after image formation. Lightfastness can be defined as that property of the ink composition which renders it resistant to a change in color when exposed to the light. Improved lightfastness is obtained with the ink composition of the present invention primarily because of the presence of an oil soluble dye.
Additionally, there is a need for ink compositions wherein the compositions are storage stable, i.e. the dyes selected are permanently retained within the vesicle and do not settle out on storage.
There is also a need for black and colored ink jet compositions with vesicles therein which can be selected for the development of images of excellent resolution on plain uncoated papers.
Moreover, there is a need for ink compositions that exhibit a high optical density which provides a measure of the quality of the appearance of the ink on paper. A reflectance (optical density) in the range of 1.0 to 1.4 is preferred. An ink composition should contain a dye content in the range of about 3-7% of the total ink composition depending on the molar extinction coefficient and molecular weight of the dye. The preferred viscosity range is low, between about 1.5 and 1.8 cps (measured against water with a value of 1) for ink jet printers using a piezo-electric driver and 2-3 cps for bubble jet printers. The preferred surface tension is approximately 55-60 dynes/cm.
In addition, other important qualities possessed by the ink compositions of the present invention include relative fast drying on the paper (within a few seconds) and water resistance after drying. Successful ink compositions are also chemically stable and have minimal settling out of particles for longer shelf-life, for example 1-2 years. If used in a bubble jet printer, the ink will be subjected to 300.degree. C. (bulk temperature approaches 100.degree. C.) and must be able to retain the qualities described above under these conditions. Finally, the ink must be safe, i.e., contain an approved dye or pigment, and should use relatively inexpensive materials and be easy to manufacture.
These characteristics, particularly the low viscosity and the water base make these ink compositions also suitable for flexographic and rotogravure printing methods. Flexography uses a single roller which achieves more controlled inking than offset and prints with a soft, shallow relief plate enabling printing on flexible materials.
The flexographic process requires solvents that do not erode rubber rollers and printing plates, thus making the water-based ink composition of the present invention ideal for this process. Further advantages of water inks in the flexographic and rotogravure processes include excellent press stability, printing quality, heat resistance, absence of fire hazard associated with solvents, and the convenience and economy of water.